1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus including the power supply apparatus.
2. Description of the Related Art
A power supply apparatus configured to output a necessary DC voltage to be used in electronic equipment is installed in various types of electronic equipment and is widely used. Electronic equipment nowadays is required to reduce power consumption in a standby mode. For this purpose, there are employed various methods for reducing power consumption in the power supply apparatus, too. For instance, in Japanese Patent Application Laid-Open No. 2010-142071, there is proposed a method in which an output voltage of a power supply circuit is lowered in the standby mode in which a load on the electronic equipment is reduced, and an oscillation period of the power supply circuit is increased so as to improve efficiency and reduce power consumption of the power supply apparatus. Japanese Patent Application Laid-Open No. 2010-142071 introduces some power supply circuits. In particular, the power supply circuit proposed in the first embodiment has a small modification from a related-art circuit structure and is easy to realize. However, in an actual power supply circuit, a protection circuit is combined in many cases as a measure against breakdown. For instance, Japanese Patent Application Laid-Open No. 2004-260977 proposes a power supply circuit in which a sub-loop (sub-feedback loop) circuit is added as the protection circuit besides the feedback loop circuit that is normally used for controlling the output voltage. In this power supply circuit, even when the feedback loop circuit, which is normally used, is broken down to be a loop open state, the sub-loop circuit as the protection circuit operates so as to safely control the output voltage.
FIG. 8 illustrates a circuit diagram incorporating a sub-loop control circuit of an AC-DC converter illustrated in FIG. 1 of Japanese Patent Application Laid-Open No. 2004-260977 into a primary to secondary conversion part of a circuit of a power supply apparatus illustrated in FIG. 1 of Japanese Patent Application Laid-Open No. 2010-142071, so as to correspond to an actual power supply circuit. A part enclosed by a broken line in FIG. 8 is the sub-loop control circuit of Japanese Patent Application Laid-Open No. 2004-260977, and the remaining circuit is the circuit of the primary to secondary conversion part of Japanese Patent Application Laid-Open No. 2010-142071. Here, operation of the circuit of FIG. 8 is described only briefly, and detailed description of FIG. 8 is given later. The circuit of the primary to secondary conversion part performs the following operation. Specifically, a shunt regulator IC12 having a reference voltage inside controls a light emission amount of an LED part of a photocoupler PC101 based on a voltage value that is obtained by dividing an output voltage Vo by resistors R105 and R106 and is input to a REF terminal. Further, a power supply IC11 controls switching of an FET Q101 based on the current amount flowing in a phototransistor part corresponding to the light emission amount of the LED part of the photocoupler PC101, so as to stabilize the output voltage Vo.
In addition, the sub-loop control circuit as an overvoltage protection circuit enclosed by the broken line performs the following operation. Specifically, a comparator IC13 compares a protection circuit operation start reference voltage Vzp generated by a zener diode VZ102 with a protection circuit reference voltage Vlp obtained by dividing the output voltage Vo by resistors R111 and R112. In the case of Vlp>Vzp, the comparator IC13 outputs a low level signal, an LED part of a photocoupler PC102 is turned on, a phototransistor part becomes conductive, and the power supply IC11 suppresses oscillation of the primary side of a transformer T101 so as to drop the output voltage Vo. In contrast, in the case of Vlp<Vzp, an output of the comparator IC13 becomes a high impedance state, the LED part of the photocoupler PC102 is turned off, the phototransistor part is turned off, oscillation of the primary side of the transformer T101 is restarted so as to raise the output voltage Vo again. This operation is repeated so that the output voltage Vo is maintained at a constant voltage. Further, a division ratio of the resistors R111 and R112 is set so that Vlp>Vzp holds when the output voltage Vo rises up to a voltage value at which a protection operation is required to be started, and hence it is possible to prevent the output voltage Vo from rising further. In this way, by providing an additional control circuit having no common component with the control circuit that is normally used, the power supply circuit can be protected more safely.
However, when the power supply circuit has the circuit structure as illustrated in FIG. 8, the following problems occur. As a first problem, when the feedback circuit (primary to secondary conversion part) that is normally used maintains the normal condition until end of use of the product (until being discarded), the control circuit enclosed by the broken line becomes a redundant feedback circuit that is not used. As a second problem, a diode D103 is disposed in the power supply circuit in order to test operation of the overvoltage protection circuit enclosed by the broken line in a delivery inspection of the circuit substrate. However, the diode D103 is a circuit element for preventing backflow current to other system when an overvoltage is applied for testing operation in the delivery inspection, and is not used except for the operation test in the delivery inspection. As a third problem, when the overvoltage protection circuit becomes out of order after the delivery inspection of the product due to a certain reason such as an impact in transportation or assembling, a user cannot notice the fact. As a result, when the product is used continuously without noticing that the overvoltage protection circuit is out of order, and when the control circuit that is normally used also becomes out of order, there is substantially no protection circuit. As a fourth problem, a circuit including a transistor Q102 and resistors R107 and R108 so as to switch the output voltage Vo is disposed at the REF terminal part of the shunt regulator IC12 as the feedback circuit (in a current path of the resistors R105 and R106). This part is the most important part for determining the accuracy of the output voltage of the power supply circuit, and variations of components used in this part significantly affect a variation of the output voltage when the product is mass-produced. When an output voltage switching circuit is disposed at the part, the increase of the number of components becomes an increase of factors of the variation. As a result, a variation of the output voltage Vo may be increased.